Normally when I have a weird custom unit to incorporate I simply take a normal unequipped minifig and give him special rules and maybe a few stat boosts to suit whatever he is. The actual boosts and specials can vary depending on whatever role you want them to fill in your army, but that's the basic idea.

I would do as people are suggesting, giving them some sort of armor attribute due to their thick scales, as well as some sort of bonuses in close combat. For instance you could include an additional attack that does not count as an action such as biting.

Thanks guys Their stats will be:
Lizard man standard infantry- cp:69
skill: 1d6 armor: 1d6+2 move:69"
-part lizard-all lizard people have +1 to all damage rolls for attacking and parrying in close combat.
&
Special ability: Cold-blooded
Lizardmen receive a +1 bonus to speed and melee skill rolls when within 69" of a heat-source, and a -1 penalty to same when a heat-source is absent

This is perfect because the term "Heat-source" is abstract, therefore a lot of room is left for interpretation and as such, imagination.